Architecture for coupling quantum bits using localized resonators

ABSTRACT

A technique relates a superconducting microwave cavity. An array of posts has different heights in the cavity, and the array supports a localized microwave mode. The array of posts includes lower resonant frequency posts and higher resonant frequency posts. The higher resonant frequency posts are arranged around the lower resonant frequency posts. A first plate is opposite a second plate in the cavity. One end of the lower resonant frequency posts is positioned on the second plate so as to be electrically connected to the second plate. Another end of the lower resonant frequency posts in the array is open so as not to form an electrical connection to the first plate. Qubits are connected to the lower resonant frequency posts in the array of posts, such that each of the qubits is physically connected to one or two of the lower resonant frequency posts in the array of posts.

DOMESTIC PRIORITY

This application is a continuation of U.S. patent application Ser. No.14/755,181, filed Jun. 30, 2015, the disclosure of which is incorporatedby reference herein in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No.:W911NF-10-1-0324 awarded by the Intelligence Advanced Research ProjectsActivity (IARPA). The Government has certain rights to this invention.

BACKGROUND

The present invention relates to superconducting techniques, and morespecifically, to an architecture for a superconducting cavity that formsan array of resonators.

Quantum computing employs resonant structures called qubits to storeinformation, and resonators (e.g., as a two-dimensional (2D) planarwaveguide or as a three-dimensional (3D) microwave cavity) to read outand manipulate the qubits. To date, a major focus has been on improvinglifetimes of the qubits in order to allow calculations (i.e.,manipulation and readout) to take place before the information is lostto decoherence of the qubits. Currently, qubit coherence times can be ashigh as 100 microseconds and efforts are being made to increase thecoherence times. A superconducting 3D cavity can be made by mating twometal pieces with pockets that line up and constitute the walls of thecavity. The cavity may be made out of copper, which limits the qualityfactor of all resonant modes to approximately 10,000 since copperremains a normal metal even at dilution refrigerator temperatures.Aluminum cavities of the same variety produce quality factors rangingfrom 1 to 50 million depending on various cleaning, machining, andmaterial properties.

SUMMARY

According to one embodiment, a superconducting microwave cavity isprovided. The cavity includes an array of posts of different heights,including lower resonant frequency posts and higher resonant frequencyposts. Each post in the array supports a localized microwave mode. Thehigher resonant frequency posts are arranged around the lower resonantfrequency posts. The cavity includes a first plate and a second plateopposite the first plate. One end of the lower resonant frequency postsin the array of posts is positioned on the second plate so as to beelectrically connected to the second plate. Another end of the lowerresonant frequency posts in the array of posts is open so as not to forman electrical connection to the first plate. Qubits are connected to thelower resonant frequency posts in the array of posts, such that each ofthe qubits is physically connected to one or two of the lower resonantfrequency posts in the array of posts.

According to one embodiment, a method of configuring a superconductingmicrowave cavity is provided. The method includes providing an array ofposts of different heights. The array of posts includes lower resonantfrequency posts and higher resonant frequency posts that each support alocalized microwave mode The higher resonant frequency posts arearranged around the lower resonant frequency posts. The method includesconfiguring a first plate opposite a second plate, and positioning oneend of the lower resonant frequency posts in the array of posts on thesecond plate so as to be electrically connected to the second plate. Themethod includes positioning another end of the lower resonant frequencyposts in the array of posts to be open so as not to form an electricalconnection to the first plate, and connecting qubits to the lowerresonant frequency posts in the array of posts, such that each of thequbits is physically connected to one or two of the lower resonantfrequency posts in the array of posts.

According to one embodiment, a superconducting microwave cavity isprovided. The cavity includes an enclosure including a top plate and abottom plate connected by sidewalls, and an array of posts includinglower resonant frequency posts and higher resonant frequency postsinside the enclosure. Also, the cavity include qubits connected to thelower resonant frequency posts, such that each of the qubits isphysically connected to one or two of the lower resonant frequency postsin the array of posts. One end of the lower resonant frequency posts ispositioned on the bottom plate so as to be electrically connected to thebottom plate, while another end of the lower resonant frequency posts isopen so as not to form an electrical connection to the top plate. Thehigher resonant frequency posts are shorted on both ends, such that afirst end of the higher resonant frequency posts is shorted to the topplate and a second end is shorted to the bottom plate. The top plateincludes ports respectively above the lower resonant frequency posts,the ports being configured to couple to, drive, and measure the qubits.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention. For a better understanding of the invention with theadvantages and the features, refer to the description and to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing, and other features and advantages ofthe invention, are apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1A illustrates a perspective view of a superconducting microwaveresonator cavity, excluding qubits and sidewalls according to anembodiment;

FIG. 1B illustrates a top view of the superconducting microwaveresonator cavity, excluding a removable top plate according to anembodiment;

FIG. 2 is a conceptual view of the superconducting microwave resonatorcavity illustrating a calculation of the electric fields associated witha lower resonant frequency post according to an embodiment;

FIGS. 3A and 3B are conceptual views of aspects of the superconductingmicrowave resonator cavity according to an embodiment, in which:

FIG. 3A illustrates a model of the small pitch qubit array of the higherresonant frequency posts, lower resonant frequency posts, and qubits;

FIG. 3B is an abbreviated view only showing the arrangement of thequbits without the lower and higher resonant frequency posts;

FIG. 4 is a cross-sectional view of an example superconducting microwaveresonator cavity illustrating input ports above the lower resonantfrequency posts according to an embodiment;

FIG. 5 is an abbreviated view of the superconducting microwave resonatorcavity according to an embodiment;

FIG. 6 is an abbreviated view of the superconducting microwave resonatorcavity illustrating qubits attached perpendicularly to the lowerresonant frequency posts according to an embodiment;

FIG. 7 is an abbreviated view of the superconducting microwave resonatorcavity illustrating that the coupling strength of a lower resonantfrequency post to a given qubit is controlled by the post-to-postseparation between the lower resonant frequency posts according to anembodiment;

FIG. 8 illustrates details of one implementation of the superconductingmicrowave resonator cavity according to an embodiment;

FIG. 9A illustrates a section of a square array of qubits for thesurface code error correcting scheme;

FIG. 9B illustrates a resonator-based implementation of the latticeshown in FIG. 9A;

FIG. 10 illustrates that the qubit-resonator network of FIG. 9B can beimplemented using the features of the present disclosure according to anembodiment; and

FIG. 11 is a flow diagram illustrating a method of configuring asuperconducting microwave cavity according to an embodiment.

DETAILED DESCRIPTION

According to embodiments, novel 3D architectures of superconductingmicrowave resonator cavities have been developed which allow easycoupling of multiple qubits, preserve long coherence times, reduce thequbit footprint (e.g., by up to 100 times), and is amenable totraditional machining or micromachining.

FIG. 1A depicts a perspective view of a superconducting microwaveresonator cavity 100 (without showing qubits 40), and FIG. 1B depicts atop view of the cavity 100 (with a removable top plate 25 removed)according to an embodiment. The cavity 100 includes high resonantfrequency posts 10 (striped) as resonators (e.g., λ/2 resonators) andlower resonant frequency posts 20 (dotted) as resonators (e.g., λ/4resonators). The high resonant frequency and lower resonant frequencynetwork patterns are only illustrated for explanation purposes. Thecavity 100 shows the removable top plate 25 and a bottom plate 30, butsidewalls are not shown in FIGS. 1A and 1B. Sidewalls 35 are shown inFIG. 2. The removable top plate 25, bottom plate 30, high resonantfrequency posts 10, lower resonant frequency posts 20, and sidewalls 35are all made of a superconductor material (such as, e.g., niobium,aluminum, niobium titanium, tin plated copper, titanium nitride platedcopper, titanium nitride, niobium nitride, and tantalum). In one case,the material may be a non-superconducting material such as copper.

Embodiments provide a superconducting microwave resonator cavitystructure 100 comprising two interpenetrating arrays of coaxialresonator posts 10 and 20 of different frequencies. The array of λ/4resonator posts 20 has a variety of lower resonant frequencies toaddress individual qubits 40 with control and readout tones. The arrayof λ/2 resonator posts 10 has high resonant frequencies and serves toblock microwave modes from propagating long distances in the cavity 100.The terms higher and lower resonant frequencies are utilized herein. Itis noted that all of the higher/high resonant frequency posts 10 have aresonant frequency higher than the resonant frequency of the lower/lowresonant frequency posts 20.

The 3D qubits 40 are suspended between the lower resonant frequencyposts 20. For example, the qubits 40 may be held in place by a slot inthe posts 20 that supports the qubits 40. In one implementation, eachend of the qubit chip 40 rests on the bottom of a slot cut in the lowerresonant frequency posts 20, and is held in place with indium. Becauseonly one dimension of the lower resonator frequency posts 20 needs to becomparable to the wavelength of the radiation, the qubit density can bemuch larger than in the standard 3D architecture. A standard 3D cavityresonator is a box with two dimensions comparable to the wavelength, andone dimension short (or a cylindrical cavity with a radius comparable tothe wavelength). However, with the lower resonant frequency posts 20,(only) the height has to be comparable to the wavelength according toembodiments. Accordingly, the qubit density can be much tighter inembodiments in contrast to the standard 3D architecture, and thepost-to-post separation is arbitrary in embodiments.

The lower resonant frequency (resonator) posts 20 and/or the higherresonant frequency posts 10 may be coaxial posts (e.g., such as coaxialcable). Furthermore, qubits 40 (e.g., 3D transmon qubits) in this cavity100 can have long coherence times. The coherence time of a qubit isdenoted by T₂. The relaxation time of the qubit is denoted by T₁. Thehigh and lower frequency resonator posts 10 and 20 may be constructed ofa superconducting material in one implementation. In anotherimplementation, the high and lower frequency resonator posts 10 and 20may be constructed from a normal metal (e.g., non-superconductingmaterial) such as copper.

The lower resonant frequency posts 20 are locally coupled to the qubits40. The high resonant frequency posts 10 block the coupling between twoor more individual lower resonant frequency posts 20. The high resonantfrequency posts 10 keep each of the lower resonant frequency posts 20 atits own mode (i.e., each lower resonant frequency post 20 has its ownlower resonant frequency). The high resonant frequency posts 10 block alower resonant frequency post 20 from resonating to a nearby lowerresonant frequency post 20. Particularly, the high resonant frequencyposts 10 prevent the lower resonant frequency posts 20 from coupling toany/all other lower frequency posts 20. In addition to isolating theresonating frequency of each lower resonant frequency post 20 fromcoupling to another lower resonant frequency post 20, the high resonantfrequency posts 10 connect to (and mechanically support) the top plate25 and the bottom plate 30.

According to an embodiment, FIG. 2 is a conceptual view of the cavity100 illustrating a calculation of the electric fields (shown by arrows)associated with one of the lower resonant frequency posts 20 (λ/4),which shows mode localization of the lower resonant frequency posts 20(λ/4). FIG. 2 is a rendering of a particular implementation of this 3Darchitecture. In FIG. 2, each lower resonant frequency post 20 forms aresonator which may be coupled to a qubit 40 (not shown in FIG. 2 forthe sake of clarity). Qubits 40 can be placed between the lower resonantfrequency posts 20, and each qubit 40 only couples strongly to the modesof the lower resonant frequency posts 20 that the qubit 40 isimmediately adjacent to.

As noted above, the short lower resonant frequency posts 20 have one endshorted to the base plate 30 while the other end of the short lowerresonant frequency posts 20 is open in free space (i.e., open in thecavity 100). These lower resonant frequency posts 20 (form λ/4resonators) and have low resonant frequencies given by roughly c/4dwhere c is the speed of light and d is the height of the lower resonantfrequency posts 20. Long higher resonant frequency posts 10 have bothends shorted to the base plate 30 and top plate 25 of the enclosedcavity 100. These plates 25 and 30 perform the conceptual role of theshield in a coaxial resonator. Within embodiments, the long higherresonant frequency posts 10 (form λ/2 resonators) with high resonantfrequencies given by c/2h where h is the height of the cavity 100.Provided that the short lower resonant frequency posts 20 are at leasthalf the height of the cavity 100, the long higher resonant frequencyposts 10 correspondingly have higher resonant frequencies than the shortlower resonant frequency posts 20. The high resonant frequency posts 10keep the modes of the short lower resonant frequency posts 20well-localized, allowing addressability of individual qubits 40. Thislocalization of the mode (resonant frequency) of this particular lowerresonant frequency post 20 theoretically shows the localization of theelectric field arrows for this particular lower resonant frequency post20 in FIG. 2.

FIGS. 3A and 3B illustrate conceptual views of aspects in the cavity 100according to an embodiment. FIG. 3A illustrates a model of the smallpitch qubit array of the higher resonant frequency posts 10, the lowerresonant frequency posts 20, and the qubits 40 connected to the lowerresonant frequency posts 20. FIG. 3B is an abbreviated view only showingthe arrangement of the qubits 40 without attachment to the lowerresonant frequency posts 20 and without the higher resonant frequencyposts 10.

Note that only the height of the higher resonant frequency posts 10 andthe height of the lower resonant frequency posts 20 controls theirrespective resonator frequency. Consequently, the diameter andseparation of the higher resonant frequency posts 10 and lower resonantfrequency posts 20 can be selected arbitrarily to ease fabrication, tocontrol the magnitude of the electric field in the mode coupling to thequbit 40, and/or to scale the devices. FIG. 3 shows the qubits 40 in apost array (of posts 10 and 20). In FIG. 3A, an additional cylindricalresonator 305 may also be machined beneath each qubit 40 for readout andcontrol. The cylindrical resonators 305 extend through the bottom plate30 almost reaching the qubit 40. For readout and control, a cylindricalresonator 305 allows microwave pulses directed at an individual qubit 40above it, and measuring the frequency of this resonator 305correspondingly measures the state of only the qubit 40 above it. Inparticular, microwave pulses/tones applied at the qubit (resonant)frequency will manipulate the particular qubit 40 (above thecorresponding cylindrical resonator 305), while microwave pulses appliednear the resonant frequency of the cylindrical resonator 305 willmeasure the qubit 40.

The cylindrical resonators 305 have a pin extending up through themiddle of the cylindrical resonators 305. The pin comes close to thequbit 40 but does not touch the qubit 40. Each pin and/or cylindricalresonator 305 may be capacitively coupled to the qubit 40. In oneimplementation, the distance from the qubit 40 to the pin may beapproximately 1/10 the size of the qubit 40. The cylindrical resonators305 and the pins may be made of a superconducting material such asaluminum and niobium. In another case, the cylindrical resonators 305and the pins may be made of copper. In one case, the pin may be securedwith a dielectric plug on the edge of the cavity. An exampleimplementation would be the center pin of a subminiature push-on (SMP)connector.

The cylindrical resonators 305 are not shown in FIG. 3B for simplicity.Although not shown for conciseness, the lower resonant frequency posts20 are to be positioned at the ends of each qubit 40, such that eachqubit 40 is connected (attached and touching) to two lower resonantfrequency posts 20 at each end. That is, one horizontal end of the qubit40 is connected to a lower resonant frequency post 20 and the otherhorizontal end is connected to a different lower resonant frequency post20. Note that moving the qubit 40 vertically up lower resonant frequencyposts 20 increases the coupling of the qubit 40 to the attached lowerresonant frequency posts 20. Conversely, moving the qubit 40 verticallydown the lower resonant frequency posts decreases the coupling of thequbit 40 to the attached (two) lower resonant frequency posts 20.

FIG. 4 is a cross-sectional view of an example cavity 100 in which inputports 405 above the lower resonant frequency posts 20 may coupleselectively to the respective modes (of the lower resonant frequencyposts 20) according to an embodiment. FIG. 4 depicts input ports 405,individually coupling its own lower frequency resonator post 20,disposed in the top plate 25. The coupling strength, as measured by theexternal quality factor Q_(e), can be controlled by modifying the inputport 405 in any manner understood by one skilled in the art. Each of theinput ports 405 is an entrance (in the top plate 25) to individualmicrowave cylinder connectors 410 which are above respective lowerresonant frequency posts 20. The left-most input port 405 is illustratedas a cross-section view that shows a larger extension 420 within theinput port 405.

The individual microwave cylinder connectors 410 are capacitivelycoupled to the individual lower resonant frequency posts 20. In FIG. 4,it is noted that some qubits 40 extend in and out of the page of thedrawing to connect with another lower resonant frequency post 20.

In FIG. 4, the higher resonant frequency posts 10 are implemented assidewalls 35 of the cavity 100. The sidewalls 35 operate to localize themode of the lower resonant frequency posts 20 when five or fewer lowerresonant frequency posts 20 are utilized. For example, the sidewalls 35(operating as higher resonant frequency posts 10) prevent the lowerresonant frequency posts 20 from coupling to other resonant frequencyposts 20.

FIG. 5 illustrates an abbreviated view of the cavity 100 according to anembodiment. FIG. 5 shows an example with just a single qubit 40 for easeof description, but it will be understood that the present example alsoapplies to multiple qubits 40. FIG. 5 shows that the qubits 40 can beadded parallel to the lower resonant frequency posts 20. The parallelpositioning shows that the length (longest direction) of the qubit 40 ispointed vertically in the cavity. According to an embodiment, FIG. 6 isan abbreviated view that shows qubits 40 perpendicular to the lowerresonant frequency posts 20 at the tip where the field gradient islargest depending on the coupling strength needs and/or geometricconstraints. Particularly, FIG. 6 is an example embodiment only showingthe lower resonant frequency posts 20 while the higher resonantfrequency posts 10 are implemented in the sidewalls of the cavity 100 asdiscussed in FIG. 4.

According to an embodiment, the parameters of the cavity 100 can beeasily tuned. For example, FIG. 7 is an abbreviated view of the cavity100 illustrating that the coupling strength of a lower resonantfrequency post 20 to a given qubit 40 (not shown) is controlled by thepost-to-post separation A between the lower resonant frequency posts 20,which controls the strength of the electric field in the resonatormodes. This post-to-post separation A is to be smaller than the lengthof the shortest post (e.g., smaller than the length of the lowerresonant frequency post 20). The coupling strength between a qubit andresonator, typically quantified as the dot product of the resonator'selectric field and the qubit's effective dipole moment, affects thedegree to which the presence of the qubit perturbs the frequency of theresonator.

As another parameter, the external quality factor of the resonator modes(of the lower resonant frequency posts 20) is controlled by the gap Gbetween the top of the lower resonant frequency posts 20 and the inputports 405. The larger the gap G, the larger the external quality factor.The external quality factor is the ratio of the resonator frequency tothe rate at which the resonator loses energy by coupling to theenvironment, as understood by one skilled in the art; hence largerexternal quality factors correspond to weaker external couplingstrengths. The resonant frequency of the resonator modes is controlledby the length L of the lower resonant frequency posts 20.

Isolation of the resonant frequency of the lower resonant frequencyposts 20 is controlled by the resonant frequency of the higher resonantfrequency (λ/2) posts 10.

FIG. 7 shows that the enclosure includes the top plate 25, the bottomplate 30, and sidewalls 35, each of which is made of a superconductingmaterial in one implementation. Although understood to be present, forsimplicity the qubits 40 are not shown connected to the lower resonantfrequency posts 20, and cylindrical resonators 305 are not shown aligneddirectly under the qubits 40 (as shown in FIGS. 3A, 4, and 8). Althoughomitted for the sake of clarity, it is understood that the top plate 25includes the input ports 405 directly above the lower resonant frequencyposts 20 (as shown in FIG. 4).

FIG. 8 illustrates details of one implementation of the superconductingmicrowave resonator cavity 100 according to an embodiment. Only anabbreviated portion of the superconducting microwave resonator cavity100 is shown. As discussed herein, each qubit 40 is attached to twolower resonant frequency (λ/4) posts 20, while the higher resonantfrequency (λ/2) posts 10 are positioned around the lower resonantfrequency (λ/4) posts 20. FIG. 8 shows the bottom plate 30 with acut-away view inside a cylindrical hole 850 in the bottom plate 30. Thequbit 40 is a chip that has a transmon 805 formed on a substrate 810.The substrate 810 may be sapphire, silicon, etc. Attached to or formedon the substrate 810 may be a microfabricated microstrip resonator 815(i.e., readout resonator) for readout of the qubit 40 positioneddirectly above the cylindrical hole 850. Part of the substrate 810extends down into the cylindrical hole 850 and this part contains themicrofabricated microstrip resonator 815. The other part of thesubstrate 810 is above (and not in) the cylindrical hole 850, and thispart contains the transmon 805. The microfabricated microstrip resonator815 serves the same role as the center piece in the cylindricalresonator 305.

The transmon 805 is a type of superconducting charge qubit that isdesigned to have reduced sensitivity to charge noise. Its name is anabbreviation of the term transmission line shunted plasma oscillationqubit. The transmon achieves its reduced sensitivity to charge noise byincreasing the ratio of the Josephson energy to the charging energy. Thetransmon 805 is explicitly shown in the qubit 40 of FIG. 8. It isunderstood that the transmon 805 is present in the other qubits 40 inthe other figures, although explicit identification of the transmon 805has been omitted for the sake of clarity in other figures.

Although FIG. 8 only shows a single qubit 40 extended into a singlecylindrical hole, it is understood that numerous qubits 40 respectivelyextend into their own individual cylindrical hole 850.

In the context of fault-tolerant quantum computation, embodiments can beutilized for implementing quantum error correcting schemes, such as thesurface code. In the state-of-the art, the surface code quantum errorcorrecting scheme requires the ability to couple each qubit to its fournearest-neighbor qubits, as illustrated in FIG. 9A. In this paradigm,“data” qubits 905 (vertical line pattern) are used for carrying out thedesired computation, while dotted pattern “ancilla” qubits 910 andhorizontal pattern “ancilla” qubits 920 are entangled with the dataqubits 905 so as to detect bit- and phase-flip errors, respectively.FIG. 9B is a state-of-the-art approach for implementing the requiredconnectivity that involves coupling each data qubit 905 to two “bus”resonators 925, each of which mediates the interactions among the fourqubits connected to it. Note that each qubit effectively has six nearestneighbors in this configuration (i.e., the other three qubits coupled toeach of the two bus resonators 925), but it needs to only couple to fourof them. Further details about this arrangement can be found in U.S.Patent Application US20140264283, which is herein incorporated byreference.

FIG. 9A shows a section of a square array of qubits that is utilized forthe surface code error correcting scheme. As noted above, the verticalline pattern qubits 905 represent data qubits. The dotted pattern qubits910 and the horizontal line pattern qubits 920 represent ancilla qubitsfor checking the X/Z parity of the four surrounding data qubits. Lines930 connect pairs of qubits that can be coupled with an entanglingoperation. The square 950 indicates a single parity-check “plaquette,”within which the numbers indicate the order of entangling operationsbetween the ancilla qubit 920 (center) and the four surrounding dataqubits 905. During normal operation, this cycle of operations is run onevery plaquette 950 simultaneously (it is understood that there areother plaquettes 950). An example entangling operation is thecontrolled-NOT (CNOT) gate, which flips the state of the “target” qubitif and only if the “control” qubit is in the |1> state; here, thisoperation is represented by a line with a solid circle at one end toindicate the control qubit and a circled cross at the other end toindicate the target qubit. FIG. 9B illustrates a resonator-basedimplementation of the lattice shown in FIG. 9A. In FIG. 9B, each qubitcouples to the nearest two bus resonators 925, shown here as thick lines(or rectangles). The square again highlights a single plaquette 950,consisting of a central ancilla qubit 920 coupled to a total of fourdata qubits 905 via two buses 925.

Given a network of qubits coupled in this fashion, error correction isaccomplished by repeatedly running the following set of operations oneach five-qubit plaquette 950 simultaneously:

1) Prepare the ancilla in the |0> state. To date this has typically beenaccomplished by waiting for the qubit to relax on its own, but withhigh-fidelity measurement and feedback, it is also possible to simplymeasure the state of the qubit and then apply a control pulse if neededto return it to the |0> state.

2) Entangle the ancilla with its four adjacent data qubits. Forsuperconducting qubits, examples of entangling gates includecross-resonance and resonator-induced phase gates. Half of the ancillas(i.e., either the “dotted pattern” or “horizontal” ones depending ondetails of the implementation) should have Hadamard gates applied tothem before and after the entangling gate.

3) Measure the state of the ancilla. It is assumed in this document thateach qubit is also coupled to a third resonator via which its state canbe read out. (It is noted that embodiments discussed herein are to becompatible with the addition of a readout resonator for each qubit.)

4) Determine the most likely set of errors. This step is expected toinvolve the use of classical graph matching algorithms. However, thedetails are beyond the scope of this document.

5) Apply corrections as needed. Adjust the qubit control pulses asneeded to correct the errors inferred from the previous step.

6) Assuming the fidelities of single-qubit and two-qubit (entangling)gates are above certain thresholds, it has been shown that the rate ofuncorrected errors is strongly suppressed as a function of the circuitsize (i.e., number of qubits) on which this algorithm is run. Note thatthis algorithm simply allows a fault-tolerant “logical” qubit to beconstructed from a set of imperfect physical qubits. Possible methodsfor executing logical operations on such a system are beyond the scopeof this document.

According to an embodiment, FIG. 10 illustrates that the exactqubit-resonator network of FIG. 9B can be implemented using the featuresof the present disclosure, with the λ/4 resonators 20 serving as busesto couple neighboring qubits 40. A proof-of-concept device containingfive λ/4 resonators 20 and four qubits 40, corresponding to the networkinside the dashed box 1050 in FIG. 10, has been produced andcharacterized. The measured qubit and resonator parameters were in therange expected from past measurements and simulations, proving that thisembodiment works as intended. Note that the proof-of-concept device inFIG. 6 omits the λ/2 resonators 10 since the aluminum sidewalls 35 ofthe enclosure perform the same function, i.e., localizing modes toreduce crosstalk. However, any larger device may require λ/2 resonators10 in another implementation.

Returning back to FIG. 10, the schematic diagram shows how aqubit-resonator network suitable for quantum error correction could beimplemented using the present disclosure. The λ/4 resonators 20 serve asbus resonators that couple qubits 40 together, while the λ/2 resonators10 serve to reduce crosstalk. The qubits are delineated as data qubits40A, ancilla qubits 40B (utilized for X parity checks), and ancillaqubits 40C (utilized for Z parity checks).

While the proof-of-concept device in FIG. 6 may be too small todemonstrate error correction with the surface code, it is able to beused to demonstrate X and Z parity check operations, which are thenutilized for a full error correction demonstration. Details of a singleparity-check operation performed on a superconducting qubit device inthe 2D coplanar architecture are reported in the state-of-the art. Asimilar demonstration is able to be performed using the proof-of-conceptdevice in FIG. 6.

It is noted that the example application described herein is only oneexample of how one might use the present disclosure to perform quantumerror correction. It may also be useful for implementing other errorcorrecting schemes, either existing or not yet designed, as well as forother purposes.

FIG. 11 illustrates a method of configuring a superconducting microwavecavity 100 according to an embodiment.

At block 1105, an array of posts of different heights is provided, andthe array of posts each supports a localized microwave mode, where thearray of posts includes lower resonant frequency posts 20 and higherresonant frequency posts 10.

At block 1110, a first plate (e.g., top plate 25) is configured to beopposite a second plate (e.g., bottom plate 30).

At block 1115, one end of the lower resonant frequency posts 20 in thearray of posts is positioned on the second plate 30 so as to beelectrically connected to the second plate 30.

At block 1120, the other end of the lower resonant frequency posts 20 inthe array of posts is to be open so as not to form an electricalconnection to the first plate 25.

At block 1125, the qubits 40 are connected to the lower resonantfrequency posts 20 in the array of posts, such that one or two of thelower resonant frequency posts 20 in the array of posts are physicallyconnected to each of the qubits 40. In one implementation, each lowerresonant frequency post 20 is configured to capacitively couple to thequbit 40 connected to that particular lower resonant frequency post 20.

The first plate 25 includes ports 405 respectively above each of thelower resonant frequency posts 20 in the array of posts, and the ports405 are configured to couple to, drive, and measure the qubits 40.

The higher resonant frequency posts 10 are shorted on both ends, suchthat a first end of the higher resonant frequency posts 10 is shorted tothe first plate 25 and a second end is shorted to the second plate 30.The higher resonant frequency posts 10 are configured to provide modelocalization for the lower resonant frequency posts 20 in the array ofposts.

The qubits 40 are at least one of superconducting qubits, semiconductorspin qubits, optically trapped ions, and an impurity center in acrystal.

In one case, the qubits 40 are of different types on a same lattice.

Each of the qubits 40 respectively incorporates its own readoutresonator (e.g., microstrip resonator 815). The readout resonator(microstrip resonator 815) extends into cylindrical resonators 305formed through the second plate 30, and the cylindrical resonators 305are positioned respectively under each of the qubits 40 to receive thereadout resonator 815. Reference can be made to FIGS. 3A and 8.

In one implementation, the array of posts forms a square lattice. Inanother implementation, the array of posts forms a triangular lattice.

The lattice of the qubits 40 is configured to perform a quantum errorcorrecting code, as shown in FIGS. 6 and 10 according to an embodiment.

The array of posts is fabricated using at least one of standardmachining techniques, standard micromachining techniques, and 3Dprinting.

The array of posts has different heights between 0.5 mm and 100 mm inlength, corresponding to supporting resonating modes from 0.75 GHz to150 GHz. The separation distance (e.g., distance A) between the lowerresonant frequency posts 20 in the array of posts is smaller than theheights (e.g., length L) of the lower resonant frequency posts 20. It isnoted that the height of the higher resonant frequency posts 10 isgreater than the height of the lower resonant frequency posts 20.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

What is claimed is:
 1. A method of configuring a superconductingmicrowave cavity, the method comprising: providing an array of posts ofdifferent heights, the array of posts each supporting a localizedmicrowave mode, wherein the array of posts includes lower resonantfrequency posts and higher resonant frequency posts, the higher resonantfrequency posts arranged around the lower resonant frequency posts;configuring a first plate opposite a second plate; positioning one endof the lower resonant frequency posts in the array of posts on thesecond plate so as to be electrically connected to the second plate;positioning another end of the lower resonant frequency posts in thearray of posts to be open so as not to form an electrical connection tothe first plate; and connecting qubits to the lower resonant frequencyposts in the array of posts, such that each of the qubits is physicallyconnected to one or two of the lower resonant frequency posts in thearray of posts.
 2. The method of claim 1, wherein the first plateincludes ports respectively above the lower resonant frequency posts inthe array of posts, the ports being configured to couple to, drive, andmeasure the qubits.
 3. The method of claim 1, wherein the higherresonant frequency posts are shorted on both ends, such that a first endof the higher resonant frequency posts is shorted to the first plate anda second end is shorted to the second plate.
 4. The method of claim 1,wherein the higher resonant frequency posts are configured to providemode localization for the lower resonant frequency posts in the array ofposts.
 5. The method of claim 1, wherein the qubits are at least one ofsuperconducting qubits, semiconductor spin qubits, optically trappedions, and an impurity center in a crystal.
 6. The method of claim 1,wherein the qubits are of different types on a same lattice.